In TO220-type packages in which the rear side of the die pad is embedded in the molding compound, it is important to accurately control the thickness of the mold compound above the rear side of the die pad. This layer of mold compound provides the electrical isolation layer for the die pad. If the thickness of the mold compound varies from the desired thickness, the product fails the electrical tests and has to be discarded.
This problem has been addressed by holding the die pad within the cavity of the transfer molding system by pins arranged above and below the die pad, as is described in U.S. Pat. No. 5,514,913, for example. As the mold compound enters the cavity, the pins are slowly retracted as the mold compound begins to cure and harden. The pins are retracted fully from the cavity as the mold compound begins to fully solidify.
This method suffers from the problem that the pins experience severe wear. As the mold compound begins to cure, its viscosity increases due to the cross-linking of the resin and silica. This creates a high friction between the mold compound and the pins which is likely to cause damage to the pins.
A further problem is that the pins are unreliable as mold compound can seep into the sleeves of the pins. The pins are then unable to extend the intended distance into the package and are also unable to be fully retracted out of the package. This results in the failure of the product as the minimum thickness of the isolation layer is not provided. Furthermore, unsightly pin marks are produced on the outside of the package.
A further disadvantage is that the process window timing and sequence of the retractable pins have to be carefully controlled to achieve a successful outcome. These parameters are difficult to optimise and accurately control. The pins also create flow turbulence when be the mould compound is injected into the cavity. This can also result in an inconsistency of the thickness of the isolation layer as well as flow marks and voids.
Due to the complexity of the process window timing and pin sequence, this process is used to mold one component position of a leadframe strip in one process step. The process is, therefore, also relatively slow and expensive.
For these and other reasons, there is a need for the present invention.